Well, you found your problem:
int age = Console.ReadLine();
If you read the documentation you will see that this returns a string.
Console.ReadLine Method[
^]
The cure for this would be to tweak the line and then do some testing.
int age = (int)Console.ReadLine();
Now you should try some text in there and see the exception it throws. Then you tweak it some more and respond appropriately. How-about a
TryParse
int age;
string entry = Console.ReadLine();
if (!int.TryParse(entry, out age)) {
Console.WriteLine("Please enter a number");
} else {
Console.WriteLine("Cool! I am two years older than you. I am " + age + 2 +".");
}
It's all up to you